He said it is a syndrome that inflicts less than 1 percent of pregnant women. It causes extreme vomiting that can to lead to a 5 to 10 percent weight loss in the mother and cause electrolyte imbalances.

It is often caused by how women react to the pregnancy hormone hCG, Atlas said. It can be a sign of twins, an abnormal pregnancy, a thyroid disorder or psychological problems. It is unclear what caused Middleton's condition.

Women with the disease are usually hospitalized to protect both the baby and the mother by treating nutrition deficiencies and electrolyte balances. The mother is put on fluids and in the worst cases given steroids intravenously.

"In those women who don’t respond to anything, doctors have to give them nutrition through an IV or tube down their nose into the stomach and beyond," Atlas said. "Those are in exterme cases."